The present invention relates generally to control of interference in land mobile FM radio communication systems and, more specifically, to an arrangement and method for controlling adjacent channel interference, or splatter, caused by frequency or phase modulating a transmitter with analog and/or digital data and which is useful in narrow-band land mobile FM radio systems.
Today's communication systems require efficient use of the crowded radio spectrum, especially in congested metropolitan areas. Typically, the channels available for land mobile communications are spaced 25 KHz or 30 KHz apart in the UHF band such that transmissions from one transmitter do not interfere with the transmissions of a transmitter on an adjacent channel. Generally, frequency (or phase) modulation is used on these channels to convey analog information, such as voice, from a transmitter to a receiver. It is also desirable to transmit data over some of these channels, usually referred to as control channels. However, when information signals are modulated onto a radio frequency (RF) carrier, a wide radio spectrum can be generated. This wide spectrum overlaps or splatters energy into adjacent channels and can result in poor system senitivity for a receiver tuned to an adjacent channel. Therefore, in order to avoid splatter, it is the task of an included audio filter and limiter to constrain the amplitude and frequency of the modulating signal such that an RF carrier so modulated produces modulation energy in the adjacent channels which is at least 55-60 dB below the level of the carrier. Most mobile radios provide better than 55-60 dB of splatter protection for conventional channel spacings. However, to achieve more efficient use of the limited radio spectrum available for land mobile communications, modern communication systems are demanding narrower channel spacings, such as 12.5 KHz. Therefore, it is evident that additional protection against splatter (or adjacent channel interference) is required to ensure that energy at the channel edges is kept below an acceptable level.
In any event, certain general techniques have been developed which singularly address these individual constraints by providing apparatus for solving one particular problem.
One known improvement utilizes a data frequency modulator with deviation control to level the amount of deviation of modulated data by utilizing a feedback loop to increase or decrease the deviation such that the residual modulation is minimized. However, this known arrangement is directed to an angle modulator for a transmitter in a radio which modulates the radio carrier or a precursor of the radio carrier to a deviation amount proportional to the data input level. This deviation amount is compared to a predetermined positive deviation frequency value when the input data bit is at one binary level and is compared to a predetermined negative deviation frequency value when the data bit is at the second binary level, thus revealing differences in the carrier deviation amount and a predetermined positive or negative deviation value when the data bit is at a "one" or a "zero" level.
A second known arrangement utilizes an improved radio transmitter modulation control circuit which increases the average power of the modulating signal, resulting in an enhanced audio signal quality since the signal-to-noise ratio is likewise increased. The improved modulation control circuitry consists of an operational amplifier having a phase-lag filter which operated in either of two modes, namely a linear mode, and an amplitude limiting mode. The amplitude limiting mode alters the phase and amplitude of harmonic signals from the operational amplifier to reduce the maximum peak-to-peak output signal to an acceptable level before applying this signal to the splatter filter. This second known arrangement permits a 2.1 dB improvement in average modulating signal power when utilized in FM radio having a maximum deviation of 5,000 Hz, and assumes a fixed width for its channel assignment. Thus, each of these arrangements addresses how to best utilize a fixed width for a channel assignment which merely attempts to best utilize the fixed channel assignment by maximizing the modulation information. Thus, the first arrangement addresses the modulation sensitivity instability problem, and the second known arrangement addresses how to enhance audio signal quality by providing a 2.1 dB improvement in average modulating signal power. Furthermore, while each of these known arrangements is useful in and of themselves, none of the above known arrangements makes use of information regarding the quality of the communications link, such that modulation parameters can be cut back during strong signal conditions when high communications link quality exists and the possibility of interfering with adjacent channel users exists.
Accordingly, there exists a need for an improved method and arrangement for adaptively controlling modulation parameters in an angle modulated transmitter of a mobile radio when operating in a system having significantly reduced channel widths and which takes into account communications link quality so that adjacent channel interference (or splatter) at the system's base station can be minimized. Furthermore, there exists a need to provide the above mentioned capability in a programmable fashion, such that the degree of cut back, or adaptive control, can be tailored to the needs of a particular communication system, whether operating at UHF or VHF frequencies.